Microwave ovens commonly employ a magnetron which operates in a continuous wave (CW) mode as their heating source. The power supply for the magnetron element is critical to its reliability and start-up performance. The primary requirements of the power supply are:
1. A high output impedance which reduces sudden load current increase by rapidly reducing the available output voltage below the threshold value of the Magnetron.
2. It must supply the heater power of the magnetron while having a high output impedance.
3. It must be able to accommodate safety features while still having high output impedance and providing heater power.
4. It should have minimum weight, volume and cost.
Power supplies which fulfill these requirements commonly utilize a voltage doubler as the output rectifier. A voltage doubler has a high output impedance due to the inherent "soft regulation" nature of a voltage doubler.
Presently used converter topologies employ some form of a fly-back variant, either quasi square-wave or resonant, with switching being performed by a single component to minimize cost. However, neither type satisfies the requirements above as optimally as possible. Thus, for a magnetron for a 700 watt RF power microwave oven, the quasi-resonant fly-back converter requires a high cost semiconductor switch which must have both a high blocking voltage capability (in excess of 1200 volts for a 220 volts AC line) and also be able to pass currents in excess of 35 amperes. Similarly, the quasi-square wave fly-back converter requires an expensive semiconductor switch of similar capability to that of the resonant variant, for example, 1000 volts blocking and 55 amperes.
Another disadvantage of prior art power supplies for the magnetron of a microwave oven is the size, weight and cost of the magnetic components. Thus, power supplies presently used for microwave ovens of 700 watts RF power and a 50/60 Hz transformer weigh about 4 kilograms. They are relatively expensive and take up considerable volume within the oven housing.
A large variety of such converter circuits is in the prior art. Thus, the patent to Yamato et al. U.S. Pat. No. 4,777,575 describes a switching power supply for providing power to the magnetron of a microwave oven in which a transformer primary winding is series connected with a single transistor switch 31 controlled by driver 81 of a control circuit 8 that receives feedback from the secondary of the transformer. Energy is stored in an inductor and is transferred during the switch-off time.
Diaz U.S. Pat. No. 4,837,670 discloses a switch mode converter with an inductive winding in series with a solid state switch operated by a control circuit 10. An output sense-circuit, taken from the secondary of the transformer, provides a feedback signal to the control circuit. A circuit which provides bias voltage to the control circuit also reduces peak power dissipation when the switch is off; no separate secondary winding is used.
Gautherin et al. U.S. Pat. No. 4,675,796 describe a switching power supply that uses a capacitor in series with a diode across the primary transformer winding so that energy stored when the switch is on is diverted to the output of the converter when the switch is off so as to provide non-dissipative snubbing. High frequency switching occurs by means of FET in series with the primary of transformer.
Kuster U.S. Pat. No. 4,561,046 describes a switching converter with a protective network that diverts energy to the output rather than dissipating stored energy through a resistor. By having the magnetizing and snubbing currents diverted to the output during the switch-off time, the main switch only conducts the reflected current and not the magnetizing and snubbing currents.
Stevens U.S. Pat. No. 4,734,636 describes a switching-line regulation circuit that avoids a snubbing network by having the switch operate when no current is passing through it. The circuit does not use a feedback control circuit from a transformer secondary. A current loop circuit is formed of an inductor in series with solid state switch and a shunting diode so that, when the switch is off, current flows through the inductor and diode. A snubbing network is provided across the switch. Output to the load is taken across a capacitor rather than from the secondary of a transformer.
In Suko U.S. Pat. No. 4,866,586, a switching arrangement employing four switches is disclosed.
In Underhill U.S. Pat. No. 4,760,324, power from a snubber circuit is also used by the driver control of a FET switch. Energy stored in a capacitor is transferred by an inductor in combination with two diodes.
The patents to Archer, U.S. Pat. No. 4,365,171, Rilly et al. U.S. Pat. No. 4,864,485 and Cohen U.S. Pat. No. 4,736,285 show other types of converters.